LeetCode 30 – Next Permutation

public void nextPermutation(int[] num) {
  int k = -1;
  for(int i = 0; i < num.length-1; i++){
    if(num[i] < num[i+1]) k = Math.max(k,i);
  }
  if(k == -1){
    reverse(num,0,num.length-1); // last permutation
  } else{
    int l = 0;
    for(int i = 0; i < num.length; i++){
      if(num[i] > num[k]) l = Math.max(l, i);
    }
    swap(num, k, l);
    reverse(num, k+1, num.length-1);
  }

}

// reverse the subarray num[a..b] of num
private void reverse(int[] num, int a, int b){
  int mid = (a+b)/2;
  for(int i = a; i <= mid; i++){
    swap(num, i, a+b-i);
  }
}

private void swap(int[] num, int a, int b){
  int tmp = num[a];
  num[a] = num[b];
  num[b] = tmp;
}